Molecular Engineering of Intensely Near-Infrared Absorbing Excited States in Highly Conjugated Oligo(porphinato)zinc-(Polypyridyl)metal(II) Supermolecules

摘要

A new series of chromophores, MPZn_n, which combine ethyne-bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II) (MPZn) and oligomeric, ethyne-bridged (porphinato)zinc(II) (PZn_n) architectures, have been synthesized and characterized, along with a series of derivatives bearing pyrrolidinyl electron-releasing groups on the ancillary terpyridine units (Pyr_mMPZn_n). Cyclic voltammetric studies, as well as NMR, electronic absorption, fluorescence, and femtosecond pump-probe transient absorption spectroscopies, have been employed to study the ground- and excited-state properties of these unusual chromophores. All of these species possess intensely absorbing excited states having large spectral bandwidth that penetrate deep in the near-infrared (NIR) energy regime. Electronic structural variation of the molecular framework shows that the excited-state absorption maximum can be extensively modulated [λ_(max)(T_1 → T_n)] (880 nm < → λ_(max) < 1126 nm), while concomitantly maintaining impressively large T_1 → T_n absorption manifold spectral bandwidth (full width at half-maximum, fwhm, ～2000-2500 cm~(-1)). Furthermore, these studies enable correlation of supermolecular electronic structure with the magnitude of the excited-state lifetime (τ_(es)) and demonstrate that this parameter can be modulated over 4 orders of magnitude (～1 ns < τ_(es) < 45 μs). Terpyridyl pyrrolidinyl substituents can be utilized to destabilize terpyridyl ligand π~* energy levels and diminish the E_(1/2) (M~(3+/2+)) value of the bis(terpyridyl)metal(II) center: such perturbations determine the relative energies of the PZn_n-derived ~1π-π~* and bis(terpyridyl)metal(II) charge-transfer states and establish whether the T_1-state wave functions of MPZn_n and Pyr_mMPZn_n species manifest the extensive electronic delocalization and charge-separated (CS) features characteristic of long-lived triplet states that absorb strongly in the NIR.